Frequency modulation network



( jl Q F STUDIO M Oct. 21, 1947. M. ZIE GLER 2,429,504

' FREQUENCY MODULATION NETWORK DISCRIM R.F. mxesa L F. v AUDIO TRANSMTTTER 6' F-M TRANSMITTER NORMAL CHANNELWIDTI fa; FREQUENCV BAND REQUIRED FOR F- M NETWORK MARC ZIEGZIZR IN V EN TOR.

Patented Oct. 21, 1947 UNITED STATES PATENT OFFICE FREQUENCY MODULATION NETWORK Marc Ziegler, Buenos Aircs, Argentina, assignor to Hartford NationalBank and Trust Company, Hartford, Conn, as'trustee Application November 9, 1943, Serial No.- 509,600

2 Claims. 1.

This invention relates to frequency modulation and more particularly to frequency modulation networks for relatively restricted areas.

In designing frequency modulation networks it has been the accepted practice to locate a plurality of frequency modulated transmitters'of-adequate power and at suitable sites so as to ensure a direct ground-wave signal of minimum field strength at substantially all points of a given area. For this purpose, the power, the sites and the frequencies of the individual transmitters are chosen so as to avoid an overlapping of their signal coverage, the aforementioned area thus being divided into a plurality of local zones each covered by one of the transmitters of the network. I

The said distribution of the transmitters in a network is determined chiefly by technical considerations regarding the propagation of radio 7 Waves and minimum signal strength required for an interference-free reception. However, the design of F. M. networks is not only atechnical but also an economical problem, and I have found a system which ofiers new propects and advantages both from economical and technical standpoints.

In my prior U. S. patent'application Serial No. 478,705, now Patent No. 2,383,359, issued Aug. 21, 1945, I have described a frequency modulation receiver in which the excursion of the inter mediate frequency wave is shrunl with respect to the frequency excursion of the incoming signal in order to permit the use of narrow-band intermediate frequency amplifiers in wide-band frequency modulation networks. The said F. M. re- 1 ceiver'also comprises afrequency stabilizing link with a discriminator of the frequency counter type described in my prior application No. 477,990, now Patent No. 2,416,078, issued Feb. 18, 1947.

I have now found that by distributing in a given area (for which a certain frequency channel has been allotted) a suitable number of wideband frequency modulated transmitters of very low power and having different but relatively close central transmitting frequencies with overlapping frequency excursion ranges, and by cophasally modulating said transmitters with the same intelligence, 9, network will be obtained, the individual transmitters of which can be received separately, provided that receivers of the type described in the above mentioned prior U. S. application are used. In the receiver used the excursion of the intermediate frequency waveshould be shrunk to such an extent that the spectrum 2. width obtained is smallerthan the separation in frequency between two adjacent central transmitting frequencies. of the network, such separation being equal at the same'time to at leastitwo times'thehighest modulation frequency to be transmitted;

Aswill'be explained hereinafter, it is the frequency shrinkage. characteristic ofthe receivers which renders feasible an interference-free re eption'ofzthesignals emittedfbyonly one ofthe transmitters; althougnthe signals of the other networkttransmitters are simultaneously present intthe'radio-frequency amplifier stages of th receiver..

In view of the absorption characteristics of high-frequency waves, it can be shown that in order to obtain a certain minimum field intensity at all points of-a given area, the radiated power required'in the'case of one central transmitter will be considerably higher than the total radiated'power of a plurality of low power transmitters conveniently located so as to give a complete signal coverage in the network area.

However, the obviousapplication of this principle, i. e. the use of a'plurality of low-power frequency modulation transmitters operating on the same central frequency, cannot be put intopractice without first solving the somewhat complicated technical problem of keeping these transmitters in absolute frequency and phase synchronism.

Since in the frequency modulation network according to this invention, the network transmitters operate on different central transmitting frequencies; the problem of transmitter synchroni zation isavoided and it is therefore possible to provide a frequency modulation network in which a requiredsignal coverage will be obtained at very low cost with relatively inexpensive equipn'lent'.

A further object and advantage of the present invention lies in the fact that the low power transmitters of the'network will provide a signal of suflicientfieldstrength in the network area Without causing interference at large distances.

A still further object of thepresent invention is to obtain, within a given area, a frequency modulation network which, although using wideband modulation, will'nevertheless occupy little more than a-channel'of normal width in the fre quency spectrum.

Other objects and'advantages of this invention will become apparent when the following description is read in conjunction with-the accompanying drawings.

In the drawings:

Fig. 1 is a diagram schematically showing a frequency modulation broadcast; network according to the present invention.

Fig. 2A is a graph illustrating the relative position of the different central transmitting frequencies, and

Fig. 2B is a graph showing the relative position of the interfering frequency spectra with respect to the frequency spectrum of the received signal.

Referring now to the figures, it can be observed that the frequency modulation network according to the invention, comprises a plurality of individual wide-band modulated transmitters I, 2, 3 and 4, conveniently distributed in the area of the zone to be covered. Said transmitters are linked to a central studio 5 through transmission lines 6 which are adjusted to obtain a cophasal modulation of the network transmitters with the same intelligence. Naturally, the intelligence may also be conveyed from the central studio to the individual transmitters of the network by radio links.

The central transmitting frequencies fol, fez, fox and i 1 of the individual transmitters I, 2, 3 and 4 respectively, are located relatively close to one another in the frequency spectrum, so that the individual frequency excursion ranges thereof overlap, as shown in the graph in Fig. 2A, the spacing between adjacent central transmitting frequencies being greater than twice the highest transmitted modulation frequency.

The resulting channel width of the network, although somewhat larger than that of a normal frequency modulation transmitter, is still considerably less than the channel width required for the four transmitters in a common frequency modulation network.

One of the frequency modulation receivers used in the network according to the invention is shown in Fig. 1, where it is indicated by the general reference 1. This receiver comprises an antenna 8 coupled to a radio-frequency amplifier stage 9 followed by a mixer stage I!) in which the incoming signal is heterodyned with the frequency of a variable local oscillator H. The intermediate frequency wave obtained is amplified in the narrow-band intermediate frequency amplifier stage l2 and is applied to a frequency discriminator I3 which transforms the frequency excursions of the intermediate frequency wave into a voltage substantially proportional to the range of said excursions and. consequently to the intelligence impressed upon the incoming signal wave. The output voltage of the discriminator is then amplified in an audio frequency amplifier stage l4 and is applied to a loud-speaker 15,

Furthermore, the receiver 1 comprises a frequency stabilizing link constituted by a reactance tube I connected to the local oscillator l I, the input terminals of said reactance tube being coupled to the output of the discriminator [3.

It has been fully explained in my prior application No. 478,705 that in frequency modulation receiver, arranged as hereinbefore described, the discriminator output voltage will contain as a modulating component the low-frequency voltage representing the intelligence, so that the local oscillator will not be stabilized with respect to a constant value of frequency but will be tied to a value equal to the instantaneous signal frequency minus the intermediate frequency. Consequently, in the receiver 1, the output voltage of the discriminator which is proportional to the frequency excursions of the incoming signal, will modulate the frequency of the local oscillator II so that the resulting intermediate-frequency wave will have less excursion than the signal frequency, and it is this frequency excursion shrinkage which makes it possible to receive wide-band transmissions with receivers using narrow-band intermediate frequency amplifiers l2.

It will be understood that in the radio frequency amplifier stage 9 of receiver 1, the signals of several of the network transmitters will be present and that these signals will be demodulated simultaneously with the desired signal frequency. For instance, if receiver 1 is tuned to the desired signal frequency .fc2 of the network transmitter 2, the frequencies fol, fee and presumably far of transmitters I, 3 and 4 respectively, will also be present at the input of the mixer stage I0 and will be demodulated together with the signal frequency fc2. However, the demodulated frequency spectra LFcl, I.Fc3 and I.Fc4 of the undesired signal frequencies fol, fell and fa; respectively, will be separated from the frequency spectrum I.Fc2 of the desired signal frequency fez by an amount equal to the difference between the adjacent central transmitting frequencies of the network.

Since all network transmitters are cophasally modulated with the same intelligence, the difference between the spectra of the interfering signals frequencies and that of the desired signal remains constant though the instantaneous frequencies will vary proportionally to the intelligence, and in view of the fact that the width of the band-pass intermediate frequency amplifier stage I2 is considerably less than the separation between said central transmitting frequencies .fci, fc2, fc3 and in, the interfering frequency spectra will be filtered out and cannot pass to the audio amplifier stage I 4 of the receiver. Thus the intelligence of the desired incoming signal will be reproduced without any interference and with a fidelity corresponding to that of the common wide-band frequency modulation networks, in which receivers with wide-band band-pass F. I. amplifiers are used.

It is to be understood that the aforementioned arrangement of the frequency modulation transmitters and receivers is shown for purposes of illustration only, and that variations and modifications may be made therein without departing from the spirit of this invention, which is covered by the appended claims.

I claim:

1. In a frequency modulation network, in combination, at least two frequency modulated transmitters having different but relatively close central transmitting frequencies with overlapping frequency excursion ranges, means for cophasally modulating said transmitters with the same intelligence, and at least one receiver comprising means for heterodyning the desired signal with a local oscillation of adjustable frequency, to obtain an intermediate frequency wave, means for converting said intermediate frequency into an audio frequency wave reproducing the intelligence carried by the desired incoming signal, and means for modulating said local oscillation with said audio frequency wave to reduce the frequency excursion of said intermediate frequency wave to a spectrum width of a value smaller than the separation of said central transmitting frequencies of the network transmitters, said separation being equal to at least twice the highest transmitted modulation frequency.

2. In a frequency modulation network, in com- 5 bination, a plurality of wide-band frequency modulated low power transmitters having different and relatively closely related central transmitting frequencies and overlapping frequency excursion ranges, a central studio provided with means for cophasally modulating said transmitters with the same intelligence; and at least one receiver comprising means for heterodyning the desired signal frequency with a local oscillation of adjustable frequency, to obtain an intermediate frequency wave, a'band-pass amplifier for amplifying said intermediate frequency wave, said band-pass amplifier having a band-width substantially less than the spacing of said central transmitting frequencies, means coupled to said intermediate band-pass amplifier for transforming said amplified intermediate frequency oscillation into an audio frequency wave reproducing the intelligence carried by the desired signal, said transforming means being coupled to means for modulating the frequency of said local oscillations by said audio frequency wave, to reduce the frequency range of said intermediate frequency 6 wave to a spectrum width of a value smaller than the spacing of said central transmitting frequencies of said network transmitters, said spacing being equal to at least twice the highest transmitted modulation frequency.

v MARC ZIEGLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS F-M Noise and Interference, by Stanford Goldman from Electronics, dated August 1941, pp. 37, 38, 39, 40, 41 and 42. 

